The communication technologies of transmitting information by a sound wave include one using an ultrasonic wave and one using an audible sound wave. The use of the audible sound wave out of the two sound waves has the following advantages. First, communication can be performed using a speaker, a microphone, etc. commercially available at present. In addition, since propagation of the sound wave undergoes absorptive attenuation due to the viscosity of media, this absorptive attenuation becomes larger in proportion to the frequency. Therefore, the audible sound wave is less attenuated than the ultrasonic wave and the communication distance can be longer by the audible sound wave than by the ultrasonic wave.
However, when communication is performed by the audible sound wave, a man hears the sound of the transmission signal and thus offensive noise is made to a man. There is thus a technology of superposing the spectrum spread transmission signal on a voice or music (cf. Patent Document 1 below). In this technology described in Patent Document 1, a frequency masking threshold is calculated using the psychoacoustic model, the transmission signal is multiplied by a spread code sequence to obtain a spread signal spread over an entire frequency band, and the spread signal is superposed so as to be not more than the masking threshold.
Patent Document 1 International Publication WO 02/45286